A high-tech, 'tailored' solution for automotive lightweighting

Growth segments are few and far between in the steel industry, but ArcelorMittal USAs Tailored Blanks Group (AMTB) has honed in on one and is making the most of it by applying the latest materials science and fabrication techniques to the automotive-frame subassembly business.

The high-tech initiative has culminated in the worlds first, five-piece, hot-stamped, laser-welded door ring and B pillar and earned AMTB AMMs 2017 Steel Excellence Award for Best Innovation  Product. The subassembly, used in the Chrysler Pacifica minivan, is the result of work conducted over 36 months and totaling 2,000 engineering hours, and some 300 design iterations. It was also a collaboration involving AMTB; FCA US, the parent company of Chrysler; and Magna Internationals Cosma International, an original equipment manufacturer (OEM)

The separate process and materials have been around a few years, but this marks the first time anyone has combined the materials and processes for a single component, Todd Baker, president of AMTB in North America, pointed out. There has been laser-welding for traditional cold stamping, and there has been hot stamping. We moved to laser-welding for hot-stamped blanks to save weight and improve crash performance.

AMTB currently operates nine facilities in North America, a total which will expand to 10 with the completion of a new plant in Detroit by the end of this year. The latest addition will lift the count to six units in the U.S., two in Canada, and two in Mexico. The plants are stand-alone facilities, rather than a production line on site at a mill complex. We try to be in close proximity to the customers, Baker said.

Steel supply counts as another accommodation for customers. Sometimes the metal comes from ArcelorMittal mills, but some material is provided by the customer through a client-resale system. As with most supply chains in the automotive sector, there is a thorough supplier validation program for both materials and fabricators.

The problem we were addressing was the stronger and stronger the steel has become, the harder it is to form the parts and create complex shapes, Baker sized up the challenge facing the ATMB Group and its collaborators. Hot stamping rectified that.

The blank is heated to 900-degrees C and transferred at temperature to the press where it is formed and cooled, taking its full strength when cool. Strength gains can be three times as much or more, say from 500 MPa to 1,500 MPa, he explained.

All this came together in 2016. The door ring comprises five pieces laser- welded together. Four are 1,500 MPa and one is more ductile at 500 MPa.

In a crash, the stronger sections in front, above, and directly below the seats protect the passengers. The more ductile section behind and beneath the passengers absorbs the energy of the impact.

What AMTB brought to the table is the ability to produce a single part that can both resist the forces but also deform in a predictable way and absorb them. Being able to use different types of steel in different gauges but all in the same part we were able to achieve weight savings with crash performance, Baker said. Its having your cake and eating it too.

Using laser ablation, the company was able to combine two high-strength steel gradesUsibor, a hot-stamping grade that supports weight reduction in advanced shapes that require higher tensile strength, and Ductibor, an energy-absorbing grade designed specifically to complement Usibor in hot-stamping applications and offer ductilityto better manage the crash energies. The development marks the first time these two steels were used together in North America.

Although the technology is applicable to just about any subassembly in almost any vehicle, the Pacifica model was chosen specifically, Baker noted. Given the CAFÉ (corporate average fuel economy) standards in force, the larger vehicles provide the most opportunity for weight savings, he said. At the time of our proposal, the mini-van was up for redesign and was a good fit for the program

We are working with OEMs to implement the hot-stamped, laser-welded door ring and B-pillar concept across other vehicles and types, Baker went on to note. The materials and process can be used throughout the body-in-white. There are a host of applications for laser welding and hot stamping in vehicle manufacture.

None of the lasers or presses was warmer than the reception AMTB management and staff extended when they got word of the award. They felt validated and proud of the hard work they had done for so long, Baker said. It was also rewarding to know that they were not just making a standard part that can help people get around. They were helping make the vehicle safer, so better for people, and also lighter with better fuel economy, so better for the environment.